The combination of diffusion burners and catalytic burners is known. It is used in a sort of mixed operation, wherein as a rule the combustion chamber is started up in a purely diffusion operation up to a defined partial load. Thereafter more and more catalytic burners are added. In connection with this it is intended to operate the combustion chamber fully catalytically when under full load. Catalytic burners are distinguished in that they remain operational even in a lean fuel-air mixture. On the other hand, however, they have certain disadvantages, such as a lack of a multi-fuel capability, slow controllability, problematic ignition and start-up.
Combustion chambers for gas turbines on a basis of pre-mixing burners are also known, for example from European Patent Publication EP-B1-29 619. A pre-mixing/pre-vaporization process at a large excess air coefficient operates between the injected fuel and the condenser air within a number of pipe-shaped elements, before the actual combustion process takes place downstream of a baffle. By means of this step it is possible to reduce the emission values of pollutants from the combustion process considerably.
Combustion with the largest possible excess air coefficient-provided, on the one hand, that the flame does still burn and, furthermore, that not too much CO is generated--therefore not only reduces the amount of pollutant NO.sub.x, but also causes the other pollutants to be kept low, namely, as already mentioned, CO, and unburned hydrocarbons. This optimization process can be forced in the direction of even lower NO.sub.x values if the space for combustion and after-reactions within the combustion chamber is made of a larger size than would be necessary for the actual combustion. This permits a choice of a larger excess air coefficient, in the course of which initially larger amounts of CO are created which, however, can be reacted further to form CO.sub.2, so that at the end the CO emissions remain small. On the other hand, only little additional NO is generated because of the large excess air. Because a plurality of pipe-shaped elements provide pre-mixing in this known combustion chamber, only a sufficient number of elements are operated with fuel during the load control so that the optimum excess air coefficient required for the respective operational phase (start-up, partial load, full load) is the result.
Other types of pre-mixing burners, wherein baffles can be omitted, are known from European Patent Publication EP-B1-0 321 809 in the form of double-cone burners.
However, all combustion chambers with pre-mixing burners present the inadequacy that, at least in the operational stages in which only a portion of the burners is operated with fuel, a close approach to the limits of flame stability is made. In fact, because of the very lean mixture and the resultant low flame temperature, the extinguishing threshold under typical gas turbine conditions will already be reached at an excess air coefficient of approximately 2.0.